The present invention relates to temporal correlation SONAR systems.
Correlation SONAR systems, which are based upon the principle of waveform invariance, transmit sonic pulses vertically downward towards an ocean bottom, detect echoes of those pulses, and calculate the velocity of a vessel based upon the distance traveled by the vessel between the transmission and reception of a first pulse and a second pulse.
Correlation SONAR systems may be water or ground speed based, and spatial or temporal based. A water SONAR device uses echoes reflected off the water beneath a vessel, whereas a ground speed SONAR device uses echoes reflected off an ocean bottom. Spatial correlation SONAR calculates the velocity of a vessel by transmitting two or more pulses towards an ocean bottom, receiving those pulses back on a planar two dimensional array of hydrophones, determining which two hydrophones correlate the best, and dividing the distance between those two hydrophones by twice the time differential between the pulses. A temporal correlation SONAR system transmits several pulses towards the ocean bottom, and receives the echoed waves at an array of two or more hydrophones. For a pair of hydrophones, the system determines which two pulses correlate the best, and calculates velocity by dividing the fixed distance between the hydrophones by twice the time differential between the two correlated pulses.
For maximum correlation to occur in a SONAR system, the ray path of an initial SONAR transmission towards an ocean bottom and its return to a hydrophone must equal the ray path of a second SONAR transmission towards the bottom and its return back to a possibly different hydrophone. As alluded to in the previous paragraph, the distance vector between the initial hydrophone and the later hydrophone for which there are equal ray paths provides the total distance traveled by a vessel between the times of the two transmissions and the times of receptions of two correlated pulses.
The present invention is a temporal correlation SONAR system that calculates the velocity and position of a vessel. A sonic transducer on the vessel transmits a first pulse towards an ocean bottom followed by a set of second pulses towards the ocean bottom. The reflection of these pulses is received at a tetrad of hydrophones located on the vessel. The system correlates the first pulse as received by a forward positioned hydrophone with one or more of the pulses in the second set as received by an aft-positioned hydrophone. This correlation produces correlation values that are input into a curve fitting algorithm. The peak of the resulting curve is determined, and the time associated with that peak is used to calculate an along-ship velocity of the vessel (by dividing the distance between the hydrophones by twice the time).
To calculate the athwart-ship velocity of the vessel, a temporal correlation is performed between both pairs of diagonally displaced hydrophones in the tetrad of hydrophones. The values from these two correlations are input into a curve fitting algorithm, and maximum correlation values at the peaks of the curves are determined. Thereafter, another curve is constructed using the maximum correlation values for both pairs of diagonally displaced hydrophones, and a maximum correlation value from the along-ship calculations. For this graph, the angle that each respective hydrophone pair makes with the along-ship axis is plotted on the abscissa, and the corresponding maximum correlation value for each pair is plotted on the ordinate. The value of the angle at the peak of this curve is used to calculate the athwart-ship velocity of the vessel (by multiplying the along-ship velocity by the tangent of the angle).
As such, the present invention is capable of producing a continuous stream of velocity data. This stream of data, in addition to providing the velocity, may be used as an alternative to Global Positioning Systems (GPS) to track the position of a vessel.
It is consequently an object of the present invention to calculate the velocity of a water navigable vessel with a temporal correlation SONAR system.